Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate to combine with hydrogen atoms to produce electricity and water; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as sequential oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes a plurality of fuel cells, and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cells in the stack. Electric current must be collected at both ends of the stack (current to and current from the stack) by components known as current collectors. Current in turn is carried between the current collectors of multiple stacks in a system, as well as being carried out of the hot environment to the power conditioning electronics and/or electrical load which operate in a much lower temperature environment, typically less than 150° C. In addition, a typical SOFC stack can generate a relatively high current, sometimes exceeding 100 amperes.
In the prior art, the electrical carriers typically are formed of metal alloys having very high melting points, such as Inconel, such that they are capable of maintaining structural integrity at any possible operating temperature of the fuel cell stack. A serious drawback of these alloys, however, is that they have relatively poor electrical conductivity, having resistivity values 100 or more times greater than those of copper or other metals having high conductivity. Thus, current carriers formed of high temperature alloys either suffer significant power loss or must be sized very large to minimize power loss. When sized as required to carry currents of which an SOFC is capable, such prior art high temperature alloy current carriers are then very expensive, heavy, difficult to fabricate, difficult to route in an SOFC system, and wastefully consumptive of space.
Conventional high-conductivity metals or alloys comprising, for example, copper, silver, and/or aluminum, are not practical either. At SOFC operating temperatures, copper is very soft such that it cannot support its own weight, which behavior is defined and used herein as being incompetent. Copper also corrodes very rapidly by oxidation at these temperatures, leading to disintegration. Aluminum is liquid. Silver is extremely soft or liquid and prohibitively expensive. In fact, there are no conventional highly-conductive materials suitable for extended service alone as a current carrier in an SOFC system at the expected temperatures of operation.
What is needed in the art is a current carrying material having high conductivity that is stable mechanically and chemically at SOFC operating temperatures.
It is a principal object of the present invention to reduce the size and cost of current carriers in an SOFC system.